Pulse-echo ranging systems, also known as time-of-flight ranging systems, are commonly used in level measurement applications for determining the distance to an object (i.e. reflective surface) by measuring how long after transmission of a pulse the echo or reflected pulse is received. Such systems typically use ultrasonic pulses or pulse microwave (radar) signals in form of bursts of acoustic or electromagnetic energy.
Pulse-echo ranging systems generally have a transducer serving the dual role of transmitting and receiving the pulses, and a signal processor for detecting and calculating the position or range of the object based on the transit times of the transmitted and reflected pulses. The transducer employed in an acoustic pulse-echo ranging system typically includes an electro-mechanical vibrating element that functions as both a transmitter and a receiver. The transducer of a microwave pulse-echo ranging system typically includes a microwave transmitter/receiver antenna. Using the same transducer for transmitting as well as receiving is advantageous because the transducer will exhibit the same gain, directional characteristics etc. in both transmit and receive modes.
U.S. Pat. Nos. 6,169,706 and 6,298,008 describe methods of pulse-echo measurements in which a profile of the return echo signal is digitized, stored in memory and the stored echo profile is analyzed to determine the temporal position of the echo on a temporal axis, thus determining the distance to an object. In most conventional applications, the leading edge of the echo profile is located by identifying a point on the leading edge that is a certain level above the valley and below the peak of the echo profile. It is also known to use the trailing edge, the peak or the center of mass of the echo profile, especially when the leading edge is masked by another echo or distorted by the ring-down oscillations of the transducer. The ring-down results from stored energy being released by the transducer after excitation, and is particularly acute when the echo pulse has a low amplitude relative to the ring down pulses of the transducer, and also when the reflective surface is close to the transducer (near range).
It has turned out that the peak portion and the trailing edge of the echo profile are susceptible to be affected by the measurement environment and the target object itself, which makes it difficult to properly locate the echo arrival time and thus decreases the accuracy of measurement. For example, when monitoring the level of a liquid with a low dielectric constant, such as gasoline, in a tank, the echo pulse from the liquid level is harshly distorted by a strong echo coming shortly afterwards from the bottom of the tank. The lower the liquid level is, the more the echo from the level will get fused with the echo from the bottom.
Another example which requires a high accuracy of measurement is open channel monitoring, in which the measured level is used to calculate a flow volume in an open channel or pipe.